Structural and Spectroscopic Characterization of Rhenium Complexes

Apr 26, 2016 - Synopsis. Redox-active ligands 2,2′-bipyridine and 2,2′:6′,2″-terpyridine (bpy)n, (tpy)n (n = 0, 1−, 2−) coordinated to a c...
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Structural and Spectroscopic Characterization of Rhenium Complexes Containing Neutral, Monoanionic, and Dianionic Ligands of 2,2′-Bipyridines and 2,2′:6,2″-Terpyridines: An Experimental and Density Functional Theory (DFT)-Computational Study Mei Wang,†,‡ Thomas Weyhermüller,† Eckhard Bill,† Shengfa Ye,*,† and Karl Wieghardt*,† †

Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, D-45470 Mülheim an der Ruhr, Germany Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, P. R. China

‡

S Supporting Information *

ABSTRACT: The molecular and electronic structures of the members of the following electron transfer series have been determined by single crystal X-ray crystallography, temperature dependent magnetic susceptibility measurements, and UV−vis− NIR and electron paramagnetic resonance spectroscopy and verified by density functional theory calculations (DFT B3LYP): [Re(Mebpy)3]n, [Re(tpy)2]n, [Re(Tp)(bpy)Cl]n (n = 2+, 1+, 0, 1−), and [Re(bpy)(CO)3]1+,0,1− (Mebpy = 4, 4′-dimethyl-2,2′bipyridine; Tp− = tris-pyrazolylborate, tpy = 2, 2′:6, 2″terpyridine). For each series we show that the average Cpy−Cpy bond length and the average C−Nchel bond distance vary in a linear fashion with the charge n of the N,N′-coordinated (bpy)n and N,N′,N″-coordinated (tpy)n ligand. Consequently, the difference Δ between these two bond lengths varies also linearly with n. Δ is shown to be a useful single marker for the oxidation level of these two heterocyclic ligands (neutral, π-radical anion, and dianion). In addition, we have synthesized and structurally as well as spectroscopically characterized the following complexes: [(cyDAB•)ReIVCl3(PPh3)]0 1, [ReIII(tpy•)Cl(PPh3)2]Cl 2, [ReIII(tpy0)2Cl](OTf)2·2Et2O 8. There are no structurally significant (experimentally detectable) π-back-bond effects of the neutral bpy0 or tpy0 ligands irrespective of the dN configuration (N = 0−7) of the central Re atom.

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typical for the (bpy)0) ligand, whereas for the corresponding monoanions [M(bpy•)(mes)2]1− Δ is ∼0.03 ± 0.01 Å, typical for a coordinated radical anion. Most importantly, both Δvalues are independent of the dN electron configuration of the central first-row metal ion; no structurally significant π-backdonation effects have been detected for (bpy0) or (bpy•)1− complexes. We have shown7 that in the series [CrIII(tbpy)3]n (n = 3+, 2+, 1+, 0) and neutral [M(tbpy)3]0 (M = Mo, W) linear correlations of the averaged Cpy−Cpy and C−Nchel distances (and, consequently, the Δ-values) and the charge n in the {(tbpy)3}n unit also exist (see Figure S2) (tbpy = 4,4′-tert-butyl2,2′-bipyridine). Regrettably, we cannot construct a similar correlation for the uncoordinated ligand 2,2′:6,2″-terpyridine (tpy) since only the structure of neutral (tpy0) has been reported a number of times.8 The average Cpy−Cpy bond distance is found at 1.49 ± 0.01 Å and the average C−N distance is at 1.35 ± 0.01 Å, yielding a Δ-value of 0.14 Å. Nevertheless, many accurate structures of transition metal compounds containing a

INTRODUCTION In 2009 Goicoechea et al.1 reported the crystal structures of some alkali-metal (Na, K) salts of 2,2′-bipyridine (bpy) monoand dianions.2 Many excellent structures of the neutral bpy molecule have also been reported.3 Thus, the metrical parameters of the neutral ligand (bpy 0), its π-radical monoanion (bpy•)1−, and its diamagnetic dianion (bpy2−)2− are now known with considerable accuracy (estimated standard deviations (e.s.d.) values of C−C and C−N bond distances are ∼0.003 Å). It is therefore significant that the Cpy−Cpy bond distance between the two pyridine moieties decreases with increasing negative charge n of (bpy)n (n = 0, 1−, 2−) whereas the average C−N bond lengths increase in a linear fashion (see Figure S1).4 We have also shown5 that the difference of the Cpy−Cpy and average C−N bond length Δ (Å) decreases linearly with decreasing charge n (0, 1−, 2−). These empirical Δ-values (0.10 ± 0.02 Å for (bpy)0, 0.05 ± 0.02 Å for (bpy•)1−, and −0.07 ± 0.02 Å for (bpy2−)2−) represent therefore a useful marker for the oxidation level of N,N′-coordinated bpy-ligands. For a series of first-row transition metal compounds [M(bpy0)(mes)2]0 (mes = 2,4,6-Me3C6H2; M = CrII, MnII, FeII, CoII, NiII),6 the average Δ-value is invariably ∼0.12 ± 0.008 Å, © XXXX American Chemical Society

Received: March 10, 2016

A

DOI: 10.1021/acs.inorgchem.6b00609 Inorg. Chem. XXXX, XXX, XXX−XXX

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tridentate neutral (tpy0) ligand or π-radical anion (tpy•)1− or a dianion (tpy2−)2− have been reported in recent years. Figure S3 shows again a linear dependence of the av. Cpy−Cpy and av. C− Nchel bond distances, respectively, on the charge n of the (tpy)n ligands in (a) six structures of [M(tpy0)2]m (M = CoII, CoI, CrIII, MnII, FeII, NiII);9 (b) three structures of [M(tpy•)2]n (M = FeII, MnII, CrIII);9 and (c) three structures of [M(tpy2−)2]p (M = TiIV, VIV, MoIV).9 The corresponding Δ-values range from 0.12 ± 0.02 Å for (tpy0), 0.08 ± 0.02 Å for (tpy•)1−, to 0.04 ± 0.02 Å for (tpy2−)2−. Similarly, the series [Cr(tpy)2]n (n = 3+, 2+, 1+, 0) and [M(tpy)2]0 (M = Mo, W) display a linear dependence of the experimental av. Cpy−Cpy and av. C−Nchel bond lengths and the difference Δ as a function of the charge n of the {(tpy)2}n unit10 (see Figure S4). Thus, in principle it is possible to determine the oxidation level of tridentate tpy ligands experimentally from high resolution single crystal X-ray structures. In this work we scrutinize published structures of complexes containing Re(bpy) and Re(tpy) moieties (bpy = 2,2′bipyridine, tpy = 2,2′:6,2″-terpyridine) where substituted derivatives of both ligands are included in order to establish experimentally the oxidation level of the ligands and the metal ions. We have synthesized 10 new complexes (Chart 1) and

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RESULTS Syntheses and Characterization of Complexes. The reaction of 1 equiv of [ReVOCl2(OEt)(Ph3P)2]0 with 1 equiv of the ligand 1,4-bis(cyclohexyl)-1,4-diazabutane and 2 equiv of KC8 in tetrahydrofuran under anaerobic conditions at 20 °C affords a black-purple solid of [Re(cyDAB)(PPh3)Cl3] (1) in 23% yield. Under anaerobic conditions, 3 equiv of 4,4′-dimethyl-2,2′bipyridine, 1 equiv of [ReCl4(thf)2] (thf = tetrahydrofuran), and 4 equiv of sodium amalgam in thf afford a black-purple solid of [Re(Mebpy)3] (3) in 58% yield. The corresponding one-electron oxidized dark purple species [Re(Mebpy)3](PF6) (4) has been obtained via one-electron oxidation of 3 with 1 equiv of ferrocenium hexafluorophosphate. Further oneelectron oxidation yields [Re(Mebpy)3](OTf)2·2CH3OH (4a). The octahedral cation [Re(tpy)2]1+ has been obtained as chloride salt 6 from [ReCl4(thf)2], 2 equiv of sodium amalgam, and 2 equiv of the neutral ligand tpy in thf solution. Attempts to grow single crystals of this salt from a diethyl ether/thf (1:1) solution which contained traces of 2,6-di-tert-butyl-4-methylphenol, H(bht), as stabilizer led to the formation of X-ray quality crystals of [Re(tpy)2][bht]·thf (5). The new species [Re(Tp)(bpy)(OCH3)]0 (7b) has been prepared via reduction of [Re(Tp)(bpy)Cl](OTf) (7a)11 with ∼2.5 equiv of sodium amalgam in methanol. If the reduction is carried out in methanol with 1 equiv of zinc amalgam, [Re(Tp)(bpy)Cl]0 7 is obtained in good yields.11 Complexes 2,12 7,117a,11 and 813 have been prepared as described in the literature. Magnetism. The magnetic molar susceptibilities of powdered samples of complexes 1, 2, 3, 4a, 6, 7, 7a, and 8 have been recorded in the range 3−300 K in a 1.0 T magnetic field with a SQUID magnetometer. The results are shown in Figure 1, and fit parameters are given in Table S1. Complexes

Chart 1. Complexes and Ligands of This Work

determined their crystal structures at 100(2) K in order to obtain accurate C−C and C−N bond distances with estimated standard deviations (e.s.d.) < 0.006 Å for the determination of the ligand oxidation level. In order to establish their electronic structures spectroscopically we measured and discuss the UV− vis−NIR electronic spectra, the magnetic properties, and the Xband electron paramagnetic resonance (EPR) spectra. In addition, we have performed density functional theoretical calculations (DFT-B3LYP). Our main interest in the study concerns the electronic structures of the members of the electron transfer series [Re(bpy)3]n (n = 3+, 2+, 1+, 0, 1−), [Re(tpy)3]m (m = 2+, 1+, 0), and [Re(Tp)(bpy)Cl]p (p = 2+, 1+, 0, 1−) where we need to establish the nature of metal vs ligand centered one-electron redox processes.

Figure 1. Temperature dependence of magnetic moments of 1 (red), 2 (black), 3 (purple), 4 (pink), 4a (magenta), 6 (cyan), 7 (green), 7a (orange), and 8 (deep blue).

1, 4, 6, and 8 display temperature-independent magnetic moments in the range 0.3−0.4 μB; they are diamagnetic materials possibly contaminated with a very small paramagnetic impurity. In contrast, complexes 2 and 4a have temperatureindependent (10−300 K) magnetic moments of ∼1.6−1.73 μB indicating an S = 1/2 ground state in both cases. The observed magnetic moment of 1.6 μB of 3 in the range 150−300 K is again typical for an S = 1/2 ground state. It decreases in the range 150 to 3 K from 1.6 to 1.1 μB which is B

DOI: 10.1021/acs.inorgchem.6b00609 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry possibly due to weak intermolecular antiferromagnetic coupling. We have not been able to reproduce the reported magnetic moment of 4.1 μB at 309 K in solution for [Re(bpy)3]0.14 Interestingly, Goswawi et al.15 have reported similar magnetic behavior for a [ReII(L•)2(CO)2]0 species where (L•)1− represents the π-radical anion of (4-chloro-2phenylazo)pyridine (L0). In both cases, a low-spin d5 Re(II) central ion couples strongly intramolecularly with one π-radical. We show here by EPR-spectroscopy of [ReII(Mebpy•)2(Mebpy0)]0 that a ligand centered S = 1/2 ground state exists. The magnetic behavior of 7 and 7a is more complicated. 7a contains an octahedral low-spin ReIII ion (d4, S = 1) and no ligand-centered π-radical. The temperature dependence of μeff (1.65 μB at 300 K decreasing monotonically to 0.25 μB at 3 K) is typical for octahedral complexes of low-spin ReIII (d4) (see for example ref 16a for [ReIIICl3(pzH)2(PPh3)] where pzH represents pyrazole) in Oh symmetry which is due to large spin−orbit coupling. The magnetic moment μeff of 7 decreases monotonically from 1.4 μB at 300 K to 0.2 μB at 3 K. We have not been able to fit these data to a situation where a central low-spin ReIII ion (S = 1) couples strongly antiferromagnetically to a ligand-centered (bpy•)1−radical anion yielding a metal-centered S = 1/2 ground state, which has been unambiguously established by its EPR spectrum recorded at 30 K (see below). The diamagnetic dication 8 contains a seven-coordinate ReIII ion: [ReIII(tpy0)2Cl]2+ (S = 0) which is typical for such species16a and has been correctly described as such in refs 13 and 23. The diamagnetism of 1 is more difficult to understand. This octahedral complex could possess one of two electronic structures: [ReIV(cyDAB•)(PPh3)Cl3]0 or [ReIII(cyDAB0)(PPh3)Cl3]0. The crystal structure and the electronic spectrum of 1 display clearly the presence of a π-radical anion (cyDAB•)1−. In order to obtain a diamagnetic ground state the central ReIV ion must possess an SRe = 1/2 ground state which couples strongly intramolecularly and antiferromagnetically with a ligand radical (SL = 1/2). In comparison, the sixcoordinate complex [ReIII(bpy0)Cl3(PPh3)]0 is paramagnetic16b and displays a μeff value of 1.7 μB at 300 K which is typical of mononuclear octahedral Re(III) complexes with a low-spin d4 configuration (S = 1). This renders the bpy ligand as neutral (bpy0) which is in excellent agreement with its crystal structure and an experimental Δ-value of 0.12 Å (see below). X-Band EPR Spectroscopy. X-band EPR spectra of rhenium complexes with an S = 1/2 ground state are very informative because they allow a clear-cut distinction to be made between a metal and a ligand-centered paramagnet. Octahedral Re centered radicals (S = 1/2) display in general very broad signals (∼100−500 mT) due to a large g-anisotropy driven by strong spin−orbit coupling and an even larger magnetic hyperfine interaction from the 185/187Re (I = 5/2, 100% abundant) nuclei.17 In contrast, the spectra of a ligandcentered Re(L•) unit with a central diamagnetic Re ion exhibit much narrower EPR signals spanning only 15−50 mT. The signals are dominated by Re superhyperfine coupling of the order ∼20 × 10−4 cm−1 (arene-1,2-dithiolate18 or α-diimine radicals19). The spectral profile can be dominated by a large nuclear quadrupole interaction which can result in perturbation of the intensity and spacing of the ΔmI = 0 (hyperfine) transitions.17

Figure 2 shows the X-band EPR spectra of 2 recorded at 10 K in CH3CN solution and its simulation (for parameters see

Figure 2. Top: X-band EPR spectrum of [Re(tpy)(PPh3)2Cl]Cl· 4MeOH (2) in MeCN solution recorded at 10 K (experimental conditions: frequency 9.65 GHz; power 0.5 mW; modulation amplitude 0.75 mT; mod. frequency 100 kHz). Experimental spectra and simulations are depicted using black and red lines, respectively. Bottom: X-band EPR spectrum of [(Tp)(bpy)Re(OCH3)]0 (7b) in thf solution recorded at 30 K (experimental conditions: frequency 9.65 GHz; power 0.5 mW; modulation amplitude 0.5 mT; mod. frequency 100 kHz).

Table 1. EPR Parameters of Complexes complex 2 Ab 7b 4a 3 Bb

ARex,y,z, 10−4 cm−1

width, mT

gx,y,z

110 26 a

227 26

630 45

2.0055 2.008

2.6415 2.008

−6.2 15 19

81.7 4 3.5

−16.7 3 1

g⊥ = 2.55, g∥ ≈ 2.0 1.9895 2.0314 giso = 2.0064 giso = 2.0064

1.7050 2.00

2.0410

∼400 ∼35 ∼310 ∼160 ∼40 ∼13

a Simulation not possible (see text). bA: [(MesDABMe)ReI(CO)3(NCCH3)]0 (MesDABMe) = N,N′-(bismesityl)-1,4-diazabutadiene, ref 19. B: [ReI(bpy•)(CO)3]0 and [ReI(bpy•)(CO)3Cl]1− (80:20 or 20:80).

Table 1). The width of the rhombic signal (∼400 mT) and the large 185/187Re hyperfine coupling constants (Aiso = 322 × 10−4 cm−1) clearly indicate that the unpaired electron resides in a metal-d orbital. This would agree with an electronic structure [(tpy•)ReIIICl (PPh3)2]1+ where a low-spin ReIII ion (d4, S = 1) couples strongly with a (tpy•)1− π-radical anion yielding the metal-centered S = 1/2 ground state. The spectrum of 7 is also shown in Figure 2. We have not attempted to simulate this spectrum, as it is contaminated with small crystallites which formed from a homogeneous thf solution upon cooling to 30 K. Nevertheless, the width of ∼310 mT of the signal indicates again a metal-centered S = 1/2 C

DOI: 10.1021/acs.inorgchem.6b00609 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ground state as in [(Tp)ReIII(bpy•)(OCH3)]0 (and/or [(Tp)ReIII(bpy•)Cl]0). The spectra of [ReII(Mebpy0)3]2+ (4a) and [Re(Mebpy)3]0 (3) recorded in acetonitrile and toluene, respectively, and their simulations are shown in Figure 3. The former is characteristic

Figure 4. X-band EPR spectrum of [ReI(bpy•)(CO)3]0 and [Re I (bpy•)(CO)3 Cl] 1− in THF solution recorded at 295 K (experimental conditions: frequency 9.65 GHz; power 2 mW; modulation amplitude 0.05 mT; mod. frequency 100 kHz). The experimental spectrum (black) and simulations for the major (in green) and minor (in blue) component and the sum of both (in red) (ratio ∼80:20) are shown.

Electronic Spectra of Complexes. Table S2 summarizes the electronic spectra of complexes 1−8. Figure 5 displays the

Figure 3. Top: X-band EPR spectrum of [Re(Mebpy)3](OTf)2· 2MeOH (4a) in MeCN solution recorded at 20 K (experimental conditions: frequency 9.65 GHz; power 0.4 mW; modulation amplitude 0.75 mT; mod. frequency 100 kHz). Bottom: X-band EPR spectrum of [Re(Mebpy)3]0 (3) in toluene solution recorded at 30 K (experimental conditions: frequency 9.65 GHz; power 0.5 mW; modulation amplitude 0.75 mT; mod. frequency 100 kHz). Experimental spectra and simulations are depicted using black and red lines, respectively. See Table 1 for the fit parameters.

for a low-spin d5 configuration rendering the Mebpy ligands neutral: [ReII(Mebpy0)3]2+. In contrast, the small width of only ∼40 mT of the spectrum of neutral [Re(Mebpy)3]0 (S = 1/2) indicates the presence of a ligand-centered radical (giso = 2.021) exhibiting small 185/187Re superhyperfine coupling (Aiso = 59 × 10−4 cm−1). This spectrum would agree with an electronic structure [Re I I ( M e bpy • ) 2 ( M e bpy 0 )] 0 (localized) or [ReII{(Mebpy3)}2−]0 (delocalized) where a central low-spin ReII ion couples intramolecularly with one ligand-based electron spin yielding the residual ligand-based radical (S = 1/2). Finally, we have recorded the X-band EPR spectrum of a thf solution of [ReI(bpy0)(CO)3Cl]0 (S = 0) which was treated with 1 equiv of KC8 at 295 K (Figure 4). As Kubiak and coworkers20 have shown, two different radicals (S = 1/2) form under these conditions which are in equilibrium (eq 1).

Figure 5. Electronic spectra of three members of the electron transfer series [Re(Mebpy)3]n (n = 2+ (4a) green (CH3CN); 1+ (4) red (CH3CN); 0 (3) black (thf)).

spectra of three members of the electron transfer series [Re{(Mebpy)3}]2+,1+,0 recorded in acetonitrile (2+, 1+), and tetrahydrofuran (0) solution at 20 °C; Figure 6 shows the spectra 7 in CH3CN, 7a (in methanol), and 7b (in thf). The spectra of 6 (thf) and 8 (CH3CN) are shown in Figure 7, and those of 1 (thf) and 2 (methanol), in Figure 8. According to Heath, Yellowlees et al.,21 it is possible to identify a coordinated (bpy•)1− π radical anion by its features in the electronic spectrum (UV−vis−NIR): “Complexes thought to contain reduced, charge-localized bpy ligand must show (i) a near infrared band near 10 000 cm−1 (1000 nm) containing three peaks (or shoulders) separated by approximately 1000 cm−1 (ε ≈ 103/ligand), (ii) a visible doublet band near 20 000 cm−1 (500 nm) (ε ≈ 0.5 × 104/ligand) and (iii) a near

thf

[Re I(bpy 0)(CO)3 Cl]0 + e ⎯⎯⎯→ [Re I(bpy •)(CO)3 Cl]1 − KC8

S=0 I

•

0

S = 1/2 −

⇌ [Re (bpy )(CO)3 ] + Cl S = 1/2

(1)

The spectrum was successfully simulated involving two radicals in a ratio of ∼80:20. The small Re superhyperfine parameters for both radicals are very similar but not identical (Table 1) and indicate the presence of a diamagnetic ReI ion (low spin d6) and a (bpy•)1− π radical anion in both cases. D

DOI: 10.1021/acs.inorgchem.6b00609 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

contains a [ReII(Mebpy0)3]2+ dication, its one electron reduced form [ReII(Mebpy•)(Mebpy0) 2]1+, and [ReII(bpy•)2(bpy0)]0. If this is true, according to Heath, Yellowlees et al.,21 the spectrum of the neutral complex should feature an intervalence ligand-to-ligand charge-transfer band at ∼3800 cm−1 (2600 nm) of low intensity. This band has been observed in the spectrum of 3 at 2380 nm and, similarly, at ∼2100 nm in the monocation 4. This feature rules out an electronic structure as [ReIII(Mebpy•)3]0 for 3 which should not exhibit the low-energy LLCT band. Similar observations have been reported for the series [CrIII(tbpy)3]3+,2,1+,0 where the di- and monocations display the LLCT band ([Cr III (bpy • )(bpy 0 ) 2 ] 2+ and [CrIII(bpy•)2(bpy0)]1+) but the trication [CrIII(bpy0)3]3+ and the neutral species [CrIII(bpy•)3]0 do not.7 The three bands near 10000 cm−1 in the spectrum of 3 and 4 have been assigned as π→π* transitions of the (bpy•)1− π-radical anion. They are the hallmark of M(bpy•) units. The spectrum of neutral 7 (Figure 6) clearly shows also the presence of a (bpy•)1− radical: [(Tp)ReIII(bpy•)Cl]0. Its oneelectron oxidized form 7a displays a spectrum with a metal-toligand CT band at 560 nm and no lower energy transitions: [(Tp)ReIII(bpy0)Cl]1+. Upon reduction of the monocation [ReIII(Tp)(bpy0)Cl]1+ in thf solution with a large excess of KC8, the monoanion [ReIII(Tp)(bpy2−)Cl]1− with presumably an (S = 1) ground state (low-spin d4) and a diamagnetic dianion (bpy2−)2− is formed. Its electronic spectrum shown in Figure 6 is in agreement with this notion (Table S2). It is remarkable that the spectrum of Kubiak’s complex [ReI(bpy•)(CO)3]0 shown in Figure 9 displays similar features

Figure 6. Electronic spectra of the three members of the electron transfer series [Re(Tp)(bpy)Cl]n (n = 1+ red (CH3OH); 0 black (CH3CN); 1− orange (thf)).

Figure 7. Electronic spectra of 6 (red) in thf and 8 (black) in CH3CN.

Figure 9. Electronic spectra of [ReI(bpy2−)(CO)3]1− (green) and [ReI(bpy•)(CO)3]0 (red) in thf at 20 °C.

as those of 3 and 4 both containing the (bpy•)1− π-radical anion (Figure 5). The three ligand based π→π* transitions are observed in the range 800−1300 nm; the doublet at ∼500 nm is now split into two transitions at 810 and 500 nm, and a near UV band at ∼400 nm is also present. It is possible to distinguish the spectra of a Re(bpy•)- and a Re(bpy2−)- moiety. The three π→π* transitions observed at 930, 1100, and 1320 nm in [ReI(bpy2−)(CO)3]1− are shifted to lower energy in comparison to in a Re(bpy•)-containing species. The same behavior is observed in the spectra of 7a and its monoanion [ReIII(Tp)(bpy2−)Cl]1− (Figure 6). The electronic spectra of the Li(tpy•)1− π-radical and of the complexes [Fe I I (tpy 0 ) 2 ] 2+ , [Fe II (tpy • )(tpy 0 )] 1+ , and [FeII(tpy•) 2]0 have been reported and assigned.22 For (tpy•)1−, intense (∼104 M−1 cm−1) bands have been observed

Figure 8. Electronic spectra of 1 (red) in thf and 2 (black) CH3OH.

ultraviolet band near 25 000 cm−1 (400 nm) (ε ≈ 1.5 × 104/ ligand).” If we apply these criteria to the bpy-containing complexes 3, 4, and 4a (Figure 5), only the spectra of the neutral complex 3 and its one-electron oxidized form, namely the monocation in 4, justify an assignment of a Re(Mebpy•)1,2 unit, respectively. The dication [Re(Mebpy)3]2+ does not meet these criteria. Here only a metal-to-ligand charge transfer band is observed at 500 nm. Therefore, it is plausible that this electron transfer series E

DOI: 10.1021/acs.inorgchem.6b00609 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Selected Bond Distances (Å) of Complexes 1−8 1

3

5

6a

7

8

2.064(2)

2.066(2) 2.078(2) 2.068(2) 2.076(2) 1.990(2) 1.995(2)

2.064(3) 2.067(3) 2.071(3) 2.074(3) 1.998(3) 1.998(3)

2.038(2)

2.106(1) 2.158(1)

1.388(3)

1.446(3)

1.447(4)

1.425(5)

1.451(2)

1.440(4)

1.382(2)

1.383(4)

1.389(3)

1.372(2) 1.366(2) 1.360(2) 1.358(2)

Re−N1

2.05(1)

2.019(2)

Re−N2

2.04(1)

2.123(2)

Re−N3(central)

a

2

2.115(2)

Re−Cl Cpy−Cpy

2.389(av.) 1.394(11)

C−Nchel.

1.318(18) 1.317(18)

2.413(1) 1.447(3) 1.456(3) 1.369(3) 1.367(3) 1.370(3) 1.373(3)

2.067(1)

Average values of two independent cations.

at 330−400, 613, 704, and 940 nm which are very similar to those observed for (bpy•)1− and serve here as markers for the presence of (tpy•)1− ligands. These have also been identified in [CrIII(tpy•)(tpy0)]2+ and [CrIII(tpy•)2]1+.10 The spectrum of the uncoordinated (tpy2−)2− dianion is not known, but the data for [MIV(tpy2−)2]0 complexes (M = Ti, V, Mo) reveal that here at least two π→π* transitions are observed at lower energy (∼1200, 1030 nm).4 The spectrum of 2 shown in Figure 8 clearly displays the transitions of a coordinated π-radical anion: [ReIII(tpy•)(PPh3)2Cl]Cl. The spectra of octahedral 6 ([Re(tpy)2]Cl) and seven-coordinated 8 ([Re(tpy)2Cl]2+) are remarkably different. Clearly 8 (Figure 7) does not show features of a coordinated (tpy•)1−. Therefore, we assign its electronic structure as [ReIII(tpy0)2Cl]2+ as the original authors have proposed in 1994.13,23 In contrast, the spectra of the octahedral monocation [Re(tpy)2]+ in 5 and 6 (Figure 7) feature characteristic bands of at least one (tpy•)1− anion: [ReIII(tpy•)2]1+ or [ReIV(tpy•)(tpy2−)]1+. Finally, the spectrum of 1 exhibits the characteristic features of a coordinated (α-diimine•)1− π-radical anion (Table S2; Figure 8).24 Crystal Structure Determinations. Table S3 summarizes crystallographic details of the structure determinations, and Table 2 gives selected bond distances. The structure of the new neutral molecule [Re(CyDAB0)Cl3(PPh3)]0 (S = 0) in crystals of 1 is shown in Figure 10. Due to the low quality of the crystals, the structure determination is of low quality (large estimated standard deviations for C−C and C−N bond distances of ∼0.018 Å despite a reasonable residual R value of 0.036). Nevertheless, it does reveal the atom connectivity correctly. The Cim−Cim bond length at 1.394(11) Å as well as the average C−Nchel bond distance at 1.32(2) Å point to the presence of an (α-DAB•)1− π-radical anion as in Kubiak’s compound [ReI(α-DAB•)(CO)3(NCCH3)]0 (S = 1/ 2) (Cim−Cim 1.41(1) Å; C−Nchel 1.33(1) Å).19 Therefore, we propose an electronic structure for 1 as [ReIV(cyDAB•)Cl3(PPh3)]0. Figure S5 displays the structure of the monocation [Re(tpy)(PPh3)2Cl]1+ (S = 1/2) in crystals of 2. This structure determination is very similar to that reported by Harman et al.12 for [Re(tpy)(PPh3)2Cl](OTf)·0.8CH2Cl2 except that our data were collected at 100(2) K, whereas Harman’s data set was collected at 193 K.12 Consequently, the e.s.d. values for the C− C and C−N bond distances of the tpy ligand are ∼0.004 Å in the present case, whereas they are ∼0.01 Å in Harman’s

Figure 10. Structure of the neutral molecule in crystals of 1 (50% thermal ellipsoids; H atoms omitted).

structure. Our improved data allow the determination of the oxidation level of the N,N′,N″-coordinated tpy ligand. The av. Cpy−Cpy distance at 1.450(3) Å and the average C−Nchel bond length at 1.370(3) Å (affording an experimental Δ-value of 0.080 Å; see Figure S3) are typical for a (tpy•)1− π radical anion. Therefore, we propose an electronic structure [ReIII(tpy•)Cl(PPh3)2]1+ for 2 and not [ReII(tpy0)Cl(PPh3)2]1+ as suggested by Harman et al.12 Figure 11 shows the structures of the neutral molecule [Re(Mebpy)3]0 in crystals of 3 and the corresponding dication in crystals of 4a. The neutral molecule 3 exhibits crystallographically imposed D3-symmetry as the dication in the structure of [Re(bpy)3][ReO4]2.25 Interestingly, the anisotropic thermal parameters of the Re atom in 3 are larger than the corresponding values in 4a, which has also been reported for the structure of the dication [Re(bpy)3]2+.25 This behavior has been interpreted (and is also proposed for the neutral molecule in 3 by us) in terms of a static disorder phenomenon. It is conceivable that a [ReII(Mebpy•)2(Mebpy0)]0 molecule is distributed over three equivalent positions in the trigonal space group R3̅c.This proposal is bolstered by the observation of an average C−Nchel distance of 1.388(3) Å in neutral 3 F

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Figure 12. Averaged d(Cpy−Cpy) and d(C−Nchel) distances in the series of [Re(Mebpy)3]m (m = 2+, 1+, 0, 1−) and [ZrIV(bpy2−)3]2−, [AlIII(bpy2−)2]2− (experimental (□) and DFT calculated (●) values) as a function of the charge n in {(bpy)3}n. The inset shows the correlation of the difference Δ, Å of the average Cpy−Cpy and C−Nchel distances (●, calculated; □, experimental) and the charge n of the {(bpy)3}n (n = 0−(−6)) unit (R2 values top to bottom: 0.971, 0.974, 0.962).

neutral (Mebpy0). For three (Mebpy•)1− anions the Cpy−Cpy distance would be 1.427 Å. Thus, the X-ray structure determination of the neutral molecule in 3 leads to a most likely electronic structure [ReII(Mebpy•)2(Mebpy0)]0. The dication in 4a consists of three crystallographically independent N,N′-coordinated, neutral (Mebpy0) ligands. There are two disordered triflate anions per dication and two molecules of methanol, but the dication does not show disorder features; consequently, the anisotropic thermal parameters are significantly smaller than those in 3 or the dication in [Re(bpy)3]2+. The structure of the monocation in [Re(bpy)3](PF6) has been reported,23 and its electronic structure has been assigned as [ReI(bpy0)3]1+ with “significant population of bipyridyl LUMO”.23 The reported average Cpy −Cpy and C−Nchel distances at 1.42(2) and 1.39(2) Å would yield a Δ-value of 0.03 Å indicative of a {(bpy)3}6− unit which is not acceptable. The structure is of very low quality (note the large e.s.d. values σ = 0.02 Å) and is hampered by static disorder problems. Its electronic structure cannot be reliably deduced from these data. As shown below the calculated geometry optimized structure affords a Δ-value of 0.081 Å indicating a {(bpy)3}1− moiety as in [ReII(bpy•)(bpy0)2]1+ (delocalized). The structure of the octahedral monocation [Re(tpy)2]1+ in crystals of 5 is shown in Figure 13. The cation consists of a central Re ion, to which two crystallographically independent tpy ligands are N,N′,N″-coordinated. The two tpy ligands are within experimental error identical; they are chemically equivalent; and their oxidation level is established by an average Cpy−Cpy bond length at 1.446(3) Å and an average C− Nchel distance at 1.382(2) Å yielding a Δav value of 0.064 Å (see Figure S3): there appears to be one mono- and one triplet dianion present ((tpy•)1− and (tpy••)2−) where the excess electron is delocalized over both ligands. The 2,6-di-tert-butyl4-methyl-phenolate anion displays C−O and ring C−C distances typical for this oxidation level (Table S4). Thus, the structure determination establishes an electronic structure for the monocation as [ReIV(tpy•)(tpy••)]1+. The structure of 6 confirms this nicely because the monocation displays identical

Figure 11. Structure of the neutral molecule in crystals of 3 (top) and of the dication in crystals of 4a (bottom) (40% thermal ellipsoids).

yielding an av. Δ-value of 0.052 Å (Table 3). This implies a value for n in {(bpy)3}n of 2− (Figure 12), which is close to the expected average value required for two (Mebpy•)1− and one Table 3. Experimental Δ-Values of Complexes 2−8 (Δ = [av. d(Cpy−Cpy) − av. d(C−Nchel)])

a

complex

av. d(Cpy−Cpy), Å

av. d(C−Nchel), Å

Δ, Åa

2 3 4a 5 6 7/7b 8

1.452(3) 1.440(4) 1.466(6) 1.446(3) 1.447(5) 1.425(5) 1.451(2)

1.370(3) 1.388(3) 1.365(5) 1.382(2) 1.383(5) 1.389(3) 1.364(2)

0.082 0.052 (0.044) 0.101 (0.095) 0.065 0.063 0.036 (0.062) 0.087

DFT calculated Δ-values are given in parentheses. G

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shows the split atom model for both species. The long Re−Cl bond length at 2.353(5) Å is typical for a ReIII−Cl bond,23 whereas the Re−O distance at 2.010 Å is characteristic for a ReIII-methoxy group. The coordinated tris(pyrazolyl)borate and 2,2′-bipyridine ligands do not display features of a static disorder. We interpret this behavior as an indication that the metrical parameters of the Re(Tp)(bpy) unit are identical in the major and minor component. The Cpy−Cpy bond length of the bpy ligand at 1.425(5) Å and the average C−Nchel distance at 1.389(3) Å (yielding a Δ-value of 0.036 Å) are characteristic of an N,N′-coordinated π-radical anion (bpy•)1− (see Figures S1 and S2). Thus, both components display electronic structures as in [Re III (Tp)(bpy • )Cl] 0 and [Re III (Tp)(bpy•)(OCH3)]0. The corresponding Cpy−Cpy and av. C− Nchel bond distances have been reported for Harman’s monocation [ReIII(Tp)(bpy0)Cl]1+ (7a) at 1.475(9) and 1.370(8) Å (Δ = 0.105 Å), respectively.11 These values are typical for the presence of a neutral bpy0 ligand. The Re−Cl bond at 2.357(2) Å is identical to that in 7. Interestingly, the average Re−N bond distances of the Re(bpy) moiety in the monocation [(Tp)ReIII(bpy0)Cl]1+ at 2.092 Å and the neutral species [(Tp)ReIII(bpy•)(OCH3)]0 decrease by 0.054 Å. This is a purely electrostatic effect where the radical anion (bpy•)1− binds more strongly to the Re(III) center than its neutral (bpy0) counterpart. The corresponding averaged three Re−N bonds of the (Tp)Re moieties are in summary slightly shorter in the monocation than in the neutral species by 0.031 Å. Since the Tp− ligands are monoanions in both cases and they are coordinated to a common Re(III) center, this effect must be ascribed to a stronger trans-effect of the (bpy•)1− ligand as compared to neutral (bpy0). In agreement with this notion is the fact the Re−NTp bonds in the trans-position to the Re−Nbpy bonds are shorter in the monocation at 2.098 Å than in the neutral species at 2.136 Å. Clearly a structural trans-effect is operative (Δ = −0.038 Å) in a neutral bpy0 versus a coordinated π-radical anion (bpy•)1−. The structure of the salt [Re(tpy)2Cl](OTf)2·2Et2O (8) has been determined in this work because the previously reported structure of the hexafluorophosphate salt [Re(tpy)2Cl](PF6)213 is of low quality which does not allow an unambiguous determination of the oxidation level of the two tridentate tpy ligands in the seven-coordinate dication (C−C, C−N bond lengths have e.s.d.’s of 0.01 Å). The same authors also report structures of similar low quality containing the dications [Re(tpy)2(OH)]2+ and [Re(tpy)2(NCS)]2+. Figure 15 shows the structure of the dication in crystals of 8. The quality of this structure is excellent (R1 = 0.018) as the small e.s.d. values of ∼0.002 Å for all C−C and C−N bonds indicate. The average Cpy−Cpy bond length is at 1.451(2) Å and the average C−Nchel bond distance is at 1.364(2) Å yielding Δav. = 0.085 Å, which is different from the previous structure (0.06 Å). Deutsch et al.13 have described these three complexes as seven-coordinate Re(III) species containing two tridentate neutral (tpy0) ligands, but their Δ-values differ significantly: in [Re(tpy)2(OH)]2+, Δ = 0.085 Å; in [Re(tpy)2Cl]2+, Δ = 0.060 Å; and in [Re(tpy)2(NCS)]2+, Δ = 0.101 Å. These data do not allow a safe assignment of the oxidation level of the tpy ligands. The dication in the structure of 8 possesses crystallographic C2 site symmetry, whereas in the previous structure the dication does not exhibit crystallographically imposed symmetry but the two tpy ligands are identical within large experimental error. According to Figure S3, a Δ-value of 0.087 Å could correspond to an n value of 1.0. For two π radicals (n = 1) one would

Figure 13. Structure of the monocation in crystals of 5 at 40% thermal probability (that of 6 is similar and not shown).

geometrical parameters (av. Cpy−Cpy 1.447(5) Å, average C− Nchel 1.383(5) Å, and Δav = 0.061 Å). We note that the average Cpy−Cpy and C−Nchel distances in 5 and 6 are identical to those found in neutral, diamagnetic [CrIII(tpy•)(tpy••)]0, yielding Δ = 0.063 Å. In this case the oxidation state of the central chromium ion has been independently established by measuring the Cr K-edge X-ray absorption spectrum.10 Figure S14 displays the linear correlation of the Δ-values of 5, 6, [Cr III (tpy• )(tpy •• )] 0, [MoIV (tpy2− ) 2 ]0 , and [W V (tpy 2−)(tpy3−)]0 vs the charge n of the {(tpy)2}n unit. Complex 7 is a cocrystallizing mixture of two neutral species with [Re(Tp)(bpy)(OCH3)]0 being the major (78%) and [Re(Tp)(bpy)Cl]0 the minor component (22%). Figure 14

Figure 14. Structure of the cocrystallizing neutral molecules [Re(Tp)(bpy)Cl]0 (22%) and [Re(Tp)(bpy)(OCH3)]0 (78%) in crystals of 7. H

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systematic fashion with the nature of the ligands X which range from halide anions Cl, Br, I, to amines, to phosphines, to a methyl anion or to other C-donor ligands, to thiolates and thioethers, or even an SnCl3− group. Note that the central rhenium ion possesses in all cases a formal +I oxidation state (in Oh symmetry the t2g6 electronic structure prevails), which in principle would be ideally suited to be involved in metal-toligand π-back-donation if the energy difference between the metal d-orbitals (dxy, dxz, dyz) and the ligand π* bpy orbital is not prohibitively large. Chisholm et al.3 have previously inferred that since a carbonyl ligand is a stronger π-acceptor than a (bpy0) ligand and that, therefore, we cannot expect to detect significant structural changes in the ReI(bpy0) unit when the donor strength of X varies (everything goes to the CO ligands). We have checked this argument by looking at ReI(bpy0) structures with four, three, and only two carbonyl ligands. In all cases shown in Table S5, the Cpy−Cpy and C−Nchel and Δvalues are observed in the range expected for an N,N′coordinated neutral bpy0 and Δ is observed in the narrow range 0.118−0.128 Å typical for uncoordinated neutral (bpy0). Even in the dinuclear complex [Re2(CO)8(bpy0)]0 which contains formally two Re0, the Δ-value is 0.115 Å. In summary, these carbonyl complexes in Table S5 display no structural variations of the N,N′-coordinated neutral 2,2′-bipyridine ligands. Only in Kubiak’s anionic species [ReI(bpy)(CO)3]1− there is a significant structural change in the Re(bpy) unit observed: the Cpy−Cpy bond distance shrinks to ∼1.39 Å (characteristic of a CC double bond) and the av. C−Nchel bond lengths increase to ∼1.405 Å (C−N single bond) yielding a Δ-value of ∼ −0.015 Å which is characteristic for an uncoordinated dianion bpy2− for which Δ is observed at ∼ −0.04 Å (see Figure S1). Based on these structural observations alone it appears to be inescapable to assign an electronic structure for this complex as closed shell [ReI(bpy2−)(CO)3]1− (S = 0). This compilation of high-quality structural data in Table S5 provides strong evidence that a Re(I) center does not display structurally significant π-back-donation into a neutral bpy0 ligand. It is therefore noteworthy that in all complexes containing higher valent Re centers (two Re(III) structures, two Re(IV), seven Re(V), and even a octahedral Re(VII)) the Re(bpy0) unit displays Δ-values in the narrow range 0.10−0.12 Å characteristic of neutral (bpy0). This indicates that an N,N′coordinated neutral bpy0 ligand displays the same characteristic geometrical features irrespective of the dN conf iguration of the central Re ion (N = 0−7). This reinforces the notion that no structurally signif icant π-back-donation in any M(bpy0) moiety has been experimentally observed.6,7 An interesting case has been discovered among these most reliable 112 hits. As shown above, in the series of diamagnetic [ReI(bpy0)(CO)3X]n+ complexes where n = 0 if X is a monoanion and n = 1+ if it is a neutral ligand, the difference Δ is invariably in the range 0.10−0.13 Å. If X is a halide anion (Cl−, Br−, I−), this holds irrespective of the nature of substituents on the bpy ligand (Δ ≈ 0.12, Table S5). It is therefore rather surprising that an apparently good X-ray structure of [Re(bpy)(CO)3(NCS)]0 produced experimental Δ-values of 0.041 and 0.056 Å for two crystallographically independent molecules.26 According to the above-mentioned linear correlations of Δ vs the charge n of the (bpy)n ligand (Figures S1 and S2), this would indicate the presence of a πradical anion (bpy•)1− and the electronic structure of this complex might have been described as [Re II (bpy • )(CO)3(NCS)]0. Interestingly, the authors of ref 26 also report

Figure 15. Structure of the monocation in crystals of 8 (40% ellipsoids).

expect Δ to be 0.080 Å and for two neutral ligands (n = 0) Δ = 0.125, whereas for one tpy0 and one (tpy•)1− ligand (n = 0.5) a Δ value of 0.10 Å may be appropriate. It is therefore not possible, even with a greatly improved structure, to determine unambiguously the oxidation level of the tpy ligands in 8, but we note that the increased Δ value of the new structure does not rule out the presence of two neutral ligands: [ReIII(tpy0)2Cl]2+ as proposed by Deutsch et al.,13 which would also be in agreement with its UV−vis spectrum (Figure 8), which does not show characteristic features of a (tpy•)1− π-radical anion. It is also in agreement with the DFT optimized structure (see below): Δcalcd = 0.094 Å. Analysis of Previously Publishes X-ray Structures of Complexes Containing a Re(bpy) Moiety. A search (up to Dec. 2014) of the Cambridge Crystallographic Data Center revealed 447 hits for complexes containing a Re(bpy) moiety. Here, no restrictions with respect to the quality of the determination had been implemented. Employing restricting filters such as (a) low R-factors